Optical light wedge



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OPTICAL LIGHT WEDGE f Original Filed July 13, 1940 2 Sheets-Sheet l M fife rrmvf f,

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OPTICAL LIGHT WEDGE Original Filed July 13, 1940 2 Sheets-Shee't 2 0/ Mag 5- mm, 1?

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7 b00705 W fiuwm/ 0 BY y y ATTORNEY Patented June 14, 1949 UNITED STATES PATENT OFFICE Divided and this application March 5, 1945, Serial No. 581,061

4 Claims.

This invention relates to optics, and more particularly to apparatus that may be utilized for spectroscopes or interferometers, or for the measurement of the wave length of light, or the like.

This application is a division of an application filed July 13, 1940, in the name of Thomas W. Sukumlyn, under Serial No. 345,361, and entitled Apparatus for optical measurements, since issued on September 4, 1945, as Patent 2,384,209, under the title of Method of producing optical wedges.

Methods of measuring wave lengths of light often involve the phenomenon of interference. An interferometer is so arranged, for this purpose, that several optical paths from a common source of light differ by an odd multiple of half wave lengths, so that the rays received neutralize each other at the point of observation. The path differences may be obtained by inserting in one path, a transparent medium of greater density than air, such as glass, which retards the transmission of the light; or by providing a longer air path for some of the beams. By appropriate adjustments, the phase difierence between the individual rays may be so set that complete interference is obtained and from these adjustments, the wave length of the light may be deduced.

These methods have been utilized with some success in the past. Since the wave length of monochromatic light is extremely small, it has not been possible to provide sufflciently thin transparent media that would add but a ,few wavelengths to the path of the rays.

It is an object of this invention to make it possible to measure the wave length of light by simpler and more exact methods.

Gratings for interferometer purposes have sometimes been made of echelon or stepped form, the optical path differences between the steps representing a very large number of wave lengths. Such echelon gratings therefore produced a considerable dispersion of the spectrum, since the order of the spectrum is a function of the path difference between the steps. It is advantageous for some purposes of spectroscopy to provide an echelon grating that produces a lower order of spectrum (obtainable by reducing the distance between the steps of the echelon), and yet obtain an order of spectrum higher than is obtainable by an ordinary difiraction grating. By the aid of 2 the present invention, the echelon grating can be so made that the differential thickness between the steps can be made very small, and the order of the spectrum can be kept low.

This invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of several embodiments of the invention. For this purpose there are shown a few forms in the drawings accompanying and forming part of the present specification. These forms will now be described in detail, illustrating the general principles of the invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Referring to the drawings:

Figure 1 is a diagram illustrating one form of apparatus that may be utilized in practicing the invention;

Fig. 2 is an enlarged fragmentary view of a portion of the apparatus illustrated in Fig. 1;

Fig. 3 is a fragmentary view of an echelon grating constructed in accordance with the invention;

Fig. 4 is a diagram of apparatus for making the optical elements in accordance with the invention;

Fig. 5 is an isometric view of an optical wedge incorporating the invention;

Fig. 6 is a view similar to Fig. 4, illustrating the apparatus for producing the optical wedge of Fig. 5;

Fig. 7 is a diagram illustrating another form of the invention;

Fig. 8 is a diagram, similar to Fig. 3, of another form of echelon grating incorporating the invention; and

Fig. 9 is an isometric view of another form of optical wedge incorporating the invention.

In the diagram of Fig. 1 a. method and means for measuring the wave length of light is illustrated. It may be assumed that monochromatic light is passed through a lens system indicated diagrammatically at i. This light is passed in a parallel beam to the lower reflecting surface 2 of a reflector 3. This reflector 3 has an aperture 4. Through this aperture 4 the eye 5 of an ob-- server may be sighted to determine when there is maximum interference between rays of light, produced as hereinafter described.

iii

The rays 1, reflected from surface 2, pass downwardly from the reflector 3 to impinge upon an optical wedge structure 8. Although these rays 1 are shown as spread substantially, it is to be understood that they are quite closely confined to a region immediately opposite the viewing aperture 4. Only those rays which are thus closely confined are of utility in the manipulation of the apparatus. The wedge structure 8 is provided with reflecting top and bottom surfaces as hereinafter explained.

The optica1 wedge structure may be formed of quartz in a manner to be hereinafter described. It is characterized by the fact that it is formed as an extremely thin wedge, the thickness of which is greatly exaggerated in the diagram. The upper and lower surfaces of this member 8 form an angle that is extremely acute; of the order of several seconds of arc. Th diagram greatly exaggerates this angle. At least some of the light incident upon the upper surface of the Wedge 8 is reflected upwardly toward the aperture 4 where the effect of the reflected light may be observed by the eye 5. In Figs. 1 and 2 the incident rays 1 are shown slightly displaced from the reflected rays 9 for the sake of clarity; it being understood, however, that these rays 9 fall substantially in line with the rays I.

The upper surface of the wedge 8 is provided with an extremely thin metallic layer II), as by evaporating a metal thereon. This layer serves partly as a reflecting layer and partly as a layer through which some light can pass downwardly through the wedge 8. The lower surface of the wedge 8 may be similarly coated with a reflected layer II from which substantially all of the light incident upon the reflector II is reflected.

The rays 9 which are reflected from the upper layer I 8 and similar rays 9 which are reflected from the lower reflecting layer II, interfere and neutralize each other, producing minimum illumination at the eye if certain conditions are fulfilled. The thickness of the wedge 8 should be such that the transmitted and returned ray is an odd number of half wave lengths behind the ray reflected from the first surface. This condition may be secured by adjustment of the wedge 8 in a direction transverse to the beam 1. The movement of the wedge 8 from a position where the reflections from the two surfaces I and II are brought from an in-phase condition to one in which they are in direct opposition, is quite large, due to the very gradual change of wedge thickness opposite the bundle of rays 1.

In order therefore to measure the wave length it is necessary merely to provide appropriate scales indicating the amount of movement required for adjusting the wedge 8 from a position where there is maximum interference to a succeeding position where there is again maximum interference. Movement of the Wedge between these two positions corresponds to an increase or decrease in the thickness of the wedge where the rays I are incident, corresponding to a half wave length or odd multiple thereof of the light being measured.

For making these measurements, the wedge 8 is Shown as pported upon a plate or support I2 mounted on a base I3 movable on a plane surface I4. For moving the base I3, a long screw I5 is shown as threaded in an ear I6 formed on a stationary support, and rotatably attached to the base I3. The screw l5 carries a manipulating wheel I1. One edge of this wheel I1 is adapted to cooperate with a stationary scale I8 that may be appropriately graduated to indicate the variations in thickness of the wedge 8 at the point where the light rays I are incident. For obtaining more accurate indications, supplemental scale marks I8 may be provided around the periphery of the wheel II, cooperating with the lower edge of scale I8.

Due to the extremely small included angle between the faces of the wedge 8, a very much greater movement of the wedge 8 is required than would be required for adjusting the wedge 8 in a direction substantially parallel to the rays I. This extremely small angle has not been capable of being produced by utilizing prior methods and apparatus. By the aid of the present invention an extremely small angle wedge 8 can be readily produced.

A mechanism for producing the wedge 8 is illustrated in Fig. 4. The plate or support I2 for the wedge 8 is shown as supported within a vacuum chamber 20 formed within a container or This vessel is shown as resting upon vessel 2I. and in sealing engagement with a base 22 through which there extends a vacuum pump connection 23. The plate I2 may first be coated with the layer II in a well understood manner. A readily vaporizable transparent material such as quartz may be placed in a container or crucible 24 located within the vacuum chamber 20. This crucible.

may be heated as by the electrica1 heating element 25 connected to an appropriate source of electrical energy 26. Control of the circuit of the heating element 25 may be accomplished by the aid of a switch 21, exterior of vessel 2I. The connections to the heating element '25 may be carried out through sealed lead-in devices 28 and 29.

The vaporized quartz is projected in a stream 30 toward the plate I2. However, a screen or shield 3| is interposed between the plate I2 and the stream 38. This screen 3I' is so arranged that it may be given a uniform movement transverse to the stream 38, so as to be gradually withdrawn from, or inserted in the stream 38 between the container 24 and the plate I2. The shield 3I may be guided for this transverse movement as on the guides 6|. Thus for example it may be connected as by a flexible element 32 to a weight 33 passing over the idler pulley 34. An-

other flexible element 35, attached to the other edge of screen 3I passes over the idler pulley 36, and on to a drum 31. This drum 3'! is rotated at a uniform rate by appropriate mechanism 38, such as clockwork or the like.

Assuming that the shield 3I is first positioned so as to shield the entire surface of the plate I2, no vaporized quartz is permitted to reach the surface of plate I2. Thereafter upon movement of the shield 3| at a uniform rate toward the right by the aid of the mechanism 38, a quartz layer 39 will be disposed upon the surface of the support I2. The left hand edge portion of this support I2 will be exposed to the evaporated quartz for the longest interval, and succeeding portions of the support I2 toward the right will be subjected to the vaporized quartz for progressively shorter intervals. If the rate of movement of the shield 3I is properly controlled in accordance with the rate of evaporation of the quartz in crucible 24, the desired extremely small angular wedge can be built up, diminishing to a very narrow edge at the right hand edge of the support I2. When this operation is completed the vacuum may be broken and the container 2| lifted from base 22. By the method describedit is possible to obtain a thin quartz fllm of the order of a few hundreds of molecules in thickness, which may be made uniform as by omitting screen 31'. This method is described in said prior patent.

The system illustrated in Fig. 4 may be utilized to produce the echelon gratings illustrated in Fig. 3 or 8. In Fig. 3 the echelon grating is formed by the transparent member 40, supported upon the support 12. In this instance the mechanism 38 is so operated as to move the shield 3| in a series of discontinuous steps, to produce the stepped formation of the upper surface of the echelon 40. The height between the steps may be made very minute, of the order of only several wave lengths of light. Accordingly, light transmitted as illustrated by the rays 4| through the grating 40 can produce a spectrum of lower order than has been possible with glass echelon gratings formed by conventional optical methods. Therefore the lines of the spectrum are not dispersed to as great an extent as in other forms of echelon gratings, and the resolving power is less. This is of advantage for some types of spectroscopic analysis. Furthermore, due to the continuity of refracting material (the thickness of which is shown greatly exaggerated in Fig. 3) there is less loss by reflection. This is due to the continuity of the material. There are no airglass surfaces except at the top and bottom.

In the form shown in Fig. 3 the light is intended to be passed through the grating 40 as well as the support I2, which may be made transparent. It is possible, however, to utilize a reflection type of grating such as illustrated in Fig. 8. Here the echelon grating 40 is shown as being overlayed with a layer 42 of reflecting material, such as sputtered metal. In this case the difference in the optical paths for the light 4| is obtained by the distance through air corresponding to the height of the steps.

The wedge 8 of Fig. 1 is shown as having plane bounding surfaces. However, it is not essential to limit the production to wedges bounded by plane surfaces. Instead, a cylindrical Wedge such as illustrated in Fig. 5 may be formed. In this case the supporting member 43 is shown as having a cylindrical surface 44. Upon this surface may be disposed the quartz wedge 45 having decreasing thickness as the lower edge of the member 43 is approached. Over this surface may be deposited if desired the semitransparent metallic coating 46.

The evaporation of quartz to produce the cylindrical wedge of Fig. 5 may be accomplished by the aid of the apparatus illustrated in Fig. 6. Here the constant speed mechanism 38 is shown as providing a constant angular motion to a circular segment 41, carrying the arcuate shield 48. The axis 49 of this shield 48 and of the segment 41 is made coincident with the axis of the cylindrical surface of the support 43. The supporting segment 41 is shown as constantly urged in a counter-clockwise direction by resilient means, such as the spiral spring 50. When the apparatus is at rest the support 41 is engaged by a stop pin 5|. If the mechanism 38 is in operation, the shield 48 is rotated at a slow rate in a clockwise direction against the force of the spring 50, permitting the building up of the optical wedge 45 on the gradually exposed surface of the support 43.

In the event that a stepped formation is desired for the surface, the mechanism 38 of Fig. 6 may be given a step-by-step motion. The resulting structure is shown in Fig. 9 and comprises a 44 of the support 43, and formed respectiveiy by arcuate surfaces 32-A of progressively differing radii. A semi-transparent metallic reflecting layer I (la may also be disposed over the face of the wedge 62 to effect the same purpose as the reflecting layers I0 and 42 on Figs. 1 and 8, respectively.

The capability of depositing extremely thin layers of quartz on a supporting surface may be made use of in connection with interference apparatus, such as illustrated in Fig. '1. In this flgure there is illustrated a support 52 which has a spherical reflecting surface 53. Upon this reflecting surface is deposited an extremely thin layer 54, of quartz by the aid of the methods hereinbefore described. A semi-transparent metallic reflecting layer 55 is shown as disposed over the quartz layer 54. The thickness of the layer 54 is shown as greatly exaggerated in Fig. '1 and may be such as to cause a half wave length difference in phase or an odd multiple thereof between the two portions of a ray reflected from the surfaces 53 and 55.

Assuming that there is a parallel beam 51 incident upon the structure in a direction parallel to its axis 56, some of the light will pass through the quartz layer 54, and will be reflected from the reflecting surface 53. The rays 58 reflected from the surface 53 converge to a common point 53 on the axis 56. The rays 60 reflected from the layer 55 also converge substantially at the same point 59. Due to the extreme thinness of the layer 54, these reflected rays 60 may be caused to interfere with the reflected rays 58. In this way the focal point 59 will appear darkened.

If the reflecting surfaces 53 and 55 are confined to a very small portion of a sphere, these interfering effects may be obtained by the aid of a uniform thickness of the layer 54.

What is claimed is:

1. An optical unit having an arcuately shaped supporting surface, and a thin transparent wedge thereon, one surface of said wedge conforming in shape and being adjacent to said arcuately shaped surface, said wedge comprising a series of steps, the thickness of each step to the said arcuately shaped surface being uniform, and adjacent steps having progressively reduced thicknesses, the difference in thickness between adjacent steps being of the order of a few wave lengths of light.

2. An optical unit having an arcuately shaped supporting surface, said surface being continuously cylindrical, and a thin transparent wedge thereon, one surface of said wedge conforming in shape and being adjacent to said cylindrical surface, said wedge comprising a series of steps, the thickness of each step to the said cylindrical surface being uniform, and adjacent steps having progressively reduced thicknesses, the difference in thickness between adjacent steps being of the order of a few wave lengths of light.

3. An optical unit having an arcuately shaped supporting surface, and a thin transparent wedge thereon, one surface of said wedge conforming in shape and being adjacent to said arcuately shaped surface, said wedge comprising a series of arcuately stepped surfaces. said adjacent stepped surfaces being progressively reduced in thickness, the difference in thickness between adjacent stepped surfaces being of the order of a few wave lengths of light, and a semi-transparent metallic reflecting layer disposed over the face of the wedge.

4. An optical unit having a supporting surface, and a thin transparent wedge thereon, one surface of said wedge conforming in shape and being adjacent to said supporting surface, said wedge comprising a series of steps, the thickness 7 of eachstep to the said supporting surface being uniform, and adjacent steps having progressively reduced thickness, the difference in'thickness between adjacent steps being of the order of a few wave lengths of light.

THOMAS W. SUKUMLYN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Kolb Dec. 28, 1937 8 Number Name Date 2,160,981 OBrien June 6, 1939 2,206,169 Elsenhut et a1. July 2, 1940 2,220,860 Blodgett Nov. 5, 1940 2,220,861 Blodgett Nov. 5, 1940 2,239,452 Williams et al Apr. 22, 1941 2,274,444 Freed Feb. 24, 1942 2,281,474 Cartwright et al. Apr. 28, 1942 2,341,827 Sukumlyn Feb. 15, 1944 FOREIGN PATENTS Number Country Date 378,429 Great Britain Aug. 9, 1932 OTHER REFERENCES Procedures in Experimental Physis by Strong, 1939 edition, pages 186 and 187.

Blodgett et 2.1., Phy. Rev., vol. 51, June 1, 1937, pgs. 971-973. 

